This invention relates to centrifugal pumps and more particularly to a centrifugal pump having an impeller with straight radially extending vanes. This type of impeller is used in applications requiring a relatively low flow and a high head of pumped liquid.
The USSR Pat. No. 918560 discloses a centrifugal pump impeller of the semi-open design having multiple radially extending vanes including a series of long vanes separated by short vanes. The short vanes are arranged in several different patterns. This patent tapers the short vanes inwardly to provide the passages between the short vanes with parallel walls giving such passages constant area sections throughout their length. In general, this is the type of pump impeller used in this invention although this invention does not include the concept of making the pumping passages of constant section along their length.
It is no usually practical to make a straight radial vane pump impeller in a fully closed design, i.e., with shrouds on both faces of the impeller, because the pump passages usually are so small that it becomes difficult to make them in a metal casting process. On the other hand, it is impractical to make this type of vane arrangement in a fully open design because there will be nothing to support the short vanes and the long vanes will be too weak to withstand the stresses present during operation. Consequently, the designer of this type of vane impeller normally provides the impeller with a semi-open design, i.e., a shroud on only one face which is normally the hub face. Providing the impeller with a semi-open design makes it easier to cast the impeller and to keep the passages clean during use, in case the material being pumped should contain debris that might clog the impeller passages.
One major problem with pump impellers of the semi-open design is that the pressure of the pumped fluid exerts a high axial thrust load on the impeller placing undesirably high loads on the bearing system for the impeller. Prior designers reduced the axial thrust on semi-open design impellers by placing pressure balancing holes in the shroud to reduce the pressure applied to the outer face of the shroud. The balancing holes were usually placed near the eye of the impeller radially inwardly of the inlets to the impeller passages because pressure balancing holes are generally more effective when placed closer to the axis of rotation and it was believed that placing balancing holes in the passages would unduly reduce the hydraulic performance of the impeller. For this reason, it is unusual for balancing holes to open into the pumping passages of an impeller.